The present invention relates to hydraulic control apparatus and method for a belt-type continuously-variable transmission, more particularly to line pressure control apparatus and method.
A Summary Preprint for SPRING CONGRESS of AUTOMOTIVE ENGINEERING EXPOSITION (20005220) discloses one hydraulic control apparatus for a belt-type continuously-variable transmission (hereinafter, CVT). In this technical document, a system is disclosed in which hydraulic pressure for being supplied to a primary pulley (hereinafter, primary pressure Pp) and/or hydraulic pressure for being supplied to a secondary pulley (hereinafter, secondary pressure Ps) are directly controlled by solenoids, as a control system for a belt-catching (belt-sandwiching) pressure namely a pulley thrust (hereinafter, Fz) at the time of a shift of CVT. Further in this document, a line pressure control apparatus is disclosed in which a line pressure (hereinafter, PL) is set by adding a predetermined margin pressure to the higher one of primary pulley pressure Pp and secondary pulley pressure Ps. On the other hand, a JATCO Technical Review (2003 No. 4 pages 17-30) discloses another hydraulic control apparatus for CVT. In this technical document, a system is disclosed in which primary pulley pressure Pp and/or secondary pulley pressure Ps are controlled by a step motor, as the control system for the belt-catching pressure (pulley thrust Fz) at the time of the shift of CVT. Further in this document, a line pressure control apparatus is disclosed in which line pressure PL serving as a base pressure for each hydraulic pressure for being supplied to the corresponding pulley (hereinafter, pulley pressure) is adjusted by a pressure regulator valve so as to keep line pressure PL highest among these three pressures PL, Pp, and Ps.
Thus in these documents, line pressure PL is always set at a pressure level higher than primary and secondary pulley pressures by margins. This setting is conducted in order to secure pulley pressures Pp and Ps capable of reliably preventing a belt slip, assuming that a pressure loss (drop) might occur due to an oil-passage resistance and the like between line pressure PL and pulley pressures Pp and Ps. The belt slip means a state where the belt is slipping against the pulley due to a shortage of the belt-catching pressure (pulley thrust Fz) relative to a torque to be transmitted.
For example, in the case where a transmission ratio Ip (=Rs/Rp) calculated by dividing a belt winding radius Rs on secondary pulley side by a belt winding radius Rp on primary pulley side is greater than 1, namely when transmission ratio (pulley ratio) Ip is in a Low side; belt winding radius Rp on primary pulley side is relatively small (Rp<Rs). From the following reasons; although primary-pulley thrust Fzp is smaller than secondary-pulley thrust Fzs in the Low side of transmission ratio Ip, the difference between Fzp and Fzs is a slight amount.
Namely as a first reason, a pulley thrust (hereinafter, denoted by Fz1) necessary to maintain the belt winding radius R requires lower amount as belt winding radius R becomes smaller. Hence, this pulley thrust Fzp1 of primary pulley side becomes smaller than a pulley thrust Fzs1 of secondary pulley side as belt winding radius Rp of primary pulley side becomes smaller (in other words, as belt winding radius Rs of secondary pulley side becomes larger).
As a second reason, a pulley thrust (hereinafter, denoted by Fz2) necessary to transmit a predetermined torque requires larger amount as belt winding radius R becomes smaller. Hence, this pulley thrust Fzp2 of primary pulley side becomes larger than a pulley thrust Fzs2 of secondary pulley side as belt winding radius Rp of primary pulley side becomes smaller (in other words, as belt winding radius Rs of secondary pulley side becomes larger).
Therefore, when considering a sum of Fz1 and Fz2, the difference between a sum Fzp (=Fzp1+Fzp2) of primary pulley side and a sum Fzs (=Fzs1+Fzs2) of secondary pulley side is small. Note that an upper limit of Fzp2 is suppressed, and hence Fzp scarcely becomes higher than Fzs in the Low side of transmission ratio Ip.
Accordingly in the Low side, secondary pressure Ps higher than primary pressure Pp is needed. Hence, line pressure PL needs to be made at least equal to or higher than secondary pressure Ps.
On the other hand, there is a fear that the belt slip is caused in the primary pulley side whose belt winding radius R is relatively small. Hence, line pressure PL having a predetermined safety factor (safety margin) relative to primary pressure Pp also needs to be set in order to prevent the belt slip.
As mentioned above, primary pressure Pp is close to secondary pressure Ps in the Low side. Therefore, in the case where the safety factor of line pressure PL which is secured against primary pressure Pp is greater than the differential pressure between Pp and Ps; line pressure PL is set to have a predetermined margin pressure also against secondary pressure Ps as a result.